1065

The Canadian Mineralogist Vol. 39, pp. 1065-1071 (2001)

THE STRUCTURE OF FENCOOPERITE: 3+ UNIQUE [Fe 3O13] PINWHEELS CROSS-CONNECTED BY [Si8O22] ISLANDS

JOEL D. GRICE¤ Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada

ABSTRACT

3+ The of fencooperite, ideally Ba6Fe 3Si8O23(CO3)2Cl3¥H2O, has been solved in the trigonal space-group P3m1, with a 10.7409(5), c 7.0955(4) Å, V 708.9(1) Å3 and Z = 1. The final residual index for observed reflections is 0.038. The uniqueness of the fencooperite structure is a result of two previously unknown fundamental building blocks (FBB). The islands of silica tetrahedra form [Si8O22] units described as double open-branched triple tetrahedra sharing vertices along [001]. The 3+ other unique FBB consists of three (Fe O5) tetragonal pyramids having one of the O atoms common to all three polyhedra, forming a pinwheel trimer [Fe3O13]. The pronounced optical absorption is attributed to this coordination complex of iron. This trimer cross-links the [Si8O22] islands to form a framework structure. The framework supports intersecting tunnels in all three crystallographic directions that are filled by Ba and Cl atoms and by H2O and CO3 groups. The structure of fencooperite is compared to that of gillespite, pellyite, titantaramellite and orthoericssonite. In gillespite and pellyite, individual polyhedra of Fe2+ in four-fold coordination with oxygen cross-link sheets and chains of silica tetrahedra, whereas the chains of Fe3+ in (titan)taramellite having six-fold coordination with oxygen cross-links island rings of silica tetrahedra. In orthoericssonite, which 3+ 3+ has the same five-fold coordination of Fe as fencooperite, the Fe polyhedra cross-link [Si2O7] groups into sheets.

Keywords: fencooperite, unique crystal structure, hydrated bariumÐiron silicateÐcarbonateÐchloride, ferric iron, five-fold coordi- nation, pinwheel.

SOMMAIRE

3+ La structure cristalline de la fencooperite, dont la formule idéale serait Ba6Fe 3Si8O23(CO3)2Cl3¥H2O, a été résolue en termes du groupe spatial trigonal P3m1, avec a 10.7409(5), c 7.0955(4) Å, V 708.9(1) Å3 et Z = 1. Le résidu final pour toutes les réflexions observées est égal à 0.038. La particularité de la structure de la fencooperite vient de la présence de deux modules structuraux non connus antérieurement. Des îlots de tétraèdres de silice forment des modules de stoechiométrie [Si8O22], que l’on peut décrire comme tétraèdres triples doublement et ouvertement branchés partageant un coin le long de [001]. L’autre module 3+ structural contient trois pyramides (Fe O5) ayant une des atomes d’oxygène en commun aux trois polyèdres, pour former un trimère en “moulinet à vent” de composition [Fe3O13]. L’absorption optique prononcée serait due à cette coordinence du fer. Ce trimère rattache les îlots [Si8O22] pour former la trame de la structure. Cette trame contient des canaux dans les trois directions cristallographiques, remplis par des atomes de Ba et de Cl et par des groupes de H2O et de CO3. La structure de la fencooperite montre des points communs avec celles de la gillespite, la pellyite, la titantaramellite et l’orthoericssonite. Dans la gillespite et la pellyite, des polyèdres individuels contenant le Fe2+ tétra-coordonné par l’oxygène rattachent les feuillets et les chaînes de tétraèdres de silice, tandis que dans la titantaramellite, les chaînes contenant le Fe3+ hexa-coordonnée par l’oxygène rattachent les anneaux d’îlots de tétraèdres de silice. Dans l’orthoericssonite, qui partage avec la fencooperite le Fe3+ en coordinence cinq, les polyèdres rattachent les groupes [Si2O7] en feuillets.

(Traduit par la Rédaction)

Mots-clés: fencooperite, structure cristalline unique, silicate–carbonate–chlorure hydraté de barium et de fer, fer ferrique, coordinence cinq, “moulinet à vent”.

¤ E-mail address: [email protected]

1065 39#4-août-01-2256-09 1065 26/10/01, 14:57 1066 THE CANADIAN MINERALOGIST

INTRODUCTION relevant to the data collection and structure determina- tion is given in Table 1. The three-dimensional data 3+ Fencooperite, ideally Ba6Fe 3Si8O23(CO3)2Cl3¥H2O, were reduced for Lorentz, polarization, and background is a recently described mineral species (Roberts et al. effects and multiply-measured reflections were aver- 2001) from Trumbull Peak, Mariposa County, Califor- aged using the Bruker program SAINT. An empirical nia. The crystal-structure analysis was essential to the plate-absorption correction was done on the basis of description of this new species, as insufficient material 3998 reflections and reduced the merging R of this data was available to determine the valence of iron, and the set from 5.35% before the absorption correction to content of H2O and CO2. These three chemical param- 4.79% after it. eters were readily determined with a solution of the crys- All calculations were done with the Siemens tal structure. SHELXTL Version 5.03 system of programs, which Dunning & Cooper (1999) described the incorporates scattering factors of neutral atoms taken of the barium silicates found at Trumbull Peak. In their from the International Tables for Crystallography, Vol. description, fencooperite is referred to as mineral C (1992). During the initial examination of the reduced UKTPÐ1, a BaÐFe silicate. Further to this label, Dunning dataset to determine a starting space-group, sixteen pos- (pers. commun. 2001) states that: “Fencooperite occurs sible space-groups were considered (eight trigonal and exclusively in one lens associated with gillespite, san- eight hexagonal). Phasing of a set of normalized struc- bornite, titantaramellite, FeÐCl analogues of ericssonite ture-factors gave a mean value |E2 Ð 1| of 0.800. This E and orthoericssonite, pyrrhotite, bigcreekite, , statistic is indicative of a non-centrosymmetric crystal barite, , a BaÐFeÐMnÐCa silicateÐphos- structure, which proved to be the correct interpretation. phate (Mineral 27 in Dunning & Cooper 1999) and with- A calculated sharpened Patterson function for space erite in a matrix. In a second lens about 20 meters group P3 (the lowest symmetry possible among the six- to the south, the only minerals identified in this quartz- teen space-groups) located the two Ba sites and ten other rich lens are walstromite, pellyite, fresnoite, bigcreekite, lighter-element sites. This model refined to R = 0.174 benitoite, titantaramellite, a hydrous silicate (mineral 21 for observed data with Fo > 4(Fo). Additional O and C in Dunning & Cooper 1999), barite and some sanborn- sites were added following a series of F synthesis ite.” In the paragenetic sequence given by Roberts et al. maps, which, in turn, reduced the R index to 0.080 (ob- (2001), the crystallization of fencooperite follows bar- served data). In the final least-squares refinement, all ite, is contemporaneous with , alforsite and likely atomic positions were refined with anisotropic-displace- titantaramellite, and precedes gillespite and . ment factors to a residual of R = 0.048 (observed data in space group P3). The program MISSYM (Le Page X-RAY CRYSTALLOGRAPHY 1987) suggested the possible presence of a mirror plane, AND CRYSTAL-STRUCTURE DETERMINATION and space group P3m1 proved to be the correct choice of symmetry; the final refinement reduced R to 0.038 Precession single-crystal photographs initially (observed data). MISSYM (Le Page 1987) also indi- showed fencooperite to be hexagonal with diffraction cated that most of the structure is consistent with space symmetry 6/mmm (Roberts et al. 2001). It was not until group P6øm2, thus it is not surprising that the diffraction the crystal-structure analysis was completed that it be- symmetry is consistent with 6/mmm. The addition of an came evident that fencooperite is trigonal (pseudo-hex- isotropic-extinction factor did not improve the results, agonal). Two sets of intensity data were collected on nor was there any evidence of merohedral twinning. The different in an attempt to improve the refine- maximum and minimum electron-densities in the final ment. In fact, there was remarkably little difference be- cycle of refinement were +2.76 and Ð1.83 eÐ/Å3. Table tween the two refinements, but the marginally better 2 contains the final positional coordinates, anisotropic- experiment is described in full here. Comparing the two displacement factors, equivalent isotropic-displacement refinements, there are exact parallels, problematic aniso- parameters and bond-valence sums, and Table 3 con- tropic-displacement parameters, disparate bond-lengths in SiO4 tetrahedra and poor bond-valence sums. These problems are without doubt a signature of the poor crys- tallinity or granular nature of fencooperite. In the final data-collection, a fragment of fencooper- ite measuring 50 40 20 m was mounted on a CCD-equipped Bruker P4 fully automated four-circle diffractometer operated at 50 kV and 40 mA. With the CCD detector, almost a full sphere of intensity data were collected out to 2 = 60° using a 120 s frame time and a crystal-to-detector distance of 40 mm. With these oper- ating conditions, no decrepitation was evident in the fi- nal analysis of the intensity standards. Information

1065 39#4-août-01-2256-09 1066 26/10/01, 14:57 THE CRYSTAL STRUCTURE OF FENCOOPERITE 1067

tains selected interatomic distances and angles. Ob- is 2.58 valence units, i.e., it is underbonded. The next- served and calculated structure-factors have been sub- nearest oxygen atom (O8) is at 3.22 Å, which does not mitted to the Depository of Unpublished Data, CISTI, increase the bond strength to any appreciable extent. The National Research Council of Canada, Ottawa, Ontario reason for the poor agreement between the calculated K1A 0S2, Canada. and the theoretical value of 3 is not clearly understood,

DESCRIPTION AND DISCUSSION OF THE STRUCTURE

There are nine cation sites in fencooperite: two cat- ion sites contain Ba, one Fe, four Si and two C. The Ba1 site is at the center of an irregular polyhedron approxi- mating an incomplete, doubly terminated hexagonal prism (Fig. 1a). This polyhedron best describes the 13- fold coordination of the Ba1 site. Eleven of the coordi- nating anions are O atoms ranging in distance from 2.660 to 3.130 Å (mean 2.978 Å) to the Ba atom. The other two anions are the Cl apices of the dipyramid at distances of approximately 3.55 Å. In contrast to this Ba site is the Ba2 site at the center of a polyhedron sim- ply described as singly terminated hexagonal prism (Fig. 1b). In this Ba-centered polyhedron with 12-fold coordination, the apex is an H2O group (denoted OW10). The 10 O anions (including OW10) have bond lengths ranging from 2.751 to 3.399 Å (mean 2.943 Å). The remainder of the polyhedron comprises two ClÐ anions at 2.992 Å. The bond-valence sums of the two Ba sites differ considerably, and neither is ideal. Refine- ment of the site occupancy of both sites reveals no re- placement by cations or vacancy. The Fe site is bonded to five oxygen atoms; four of these define a near-per- fect square at a bond distance average of 2.024 Å, the fifth oxygen atom completes a square or tetragonal py- ramidal polyhedron with a considerably shorter bond- length, 1.934 Å. Using the bond-valence parameters of Brese & O’Keeffe (1991), the bond strength of the iron

1065 39#4-août-01-2256-09 1067 26/10/01, 14:57 1068 THE CANADIAN MINERALOGIST

FIG. 1. The Ba site coordinations in fencooperite. An oblique projection approximately along [110] with [001] vertical: (a) the Ba1 site, and (b) the Ba2 site.

FIG. 2. The fundamental building blocks (FBB) in fencooperite. (a) The [Si8O22] island unit in oblique projection approximately along [110], with [001] vertical. (b) The 3+ [Fe 3O13] pinwheel trimer in oblique projection approximately along [001], with [010] horizontal.

Ð but there may be factors such as covalent bonding and indicate any contribution from a (HCO3) ion, so one the uniqueness of the trimer complex described below might assume that the proton is associated with O7. Here that are not considered in the calculation. The only other again, in the IR spectrum, there is only a broad shallow mineral known to contain this rather unique coordina- band in the OÐH stretching region, which does not sup- 3+ Ð tion for Fe is orthoericssonite (Matsubara 1980). In port a model for more OH anions. The four SiO4 tetra- that structure, the mean Fe3+–O distance is 1.960 Å, hedra are somewhat irregular, with SiÐO bond lengths which is somewhat shorter than the 2.006 Å of varying from 1.55 to 1.67 Å (1.619 Å is the overall av- fencooperite, but 10% of the site is occupied by Ti4+, erage for silicate tetrahedra), and OÐSiÐO bond angles which would shorten the bond lengths. If in fact the varying from 106.7 to 115.3° (the ideal is 109.47°). The square pyramidal site contains both Fe3+ and Fe2+, the two carbonate groups are regular, trigonal planar. residual positive charge could be attained by additional The uniqueness of the fencooperite structure is a re- H+ cations. The infrared-absorption spectrum (Roberts sult of two previously undiscovered fundamental build- et al. 2001) and the bond-valence calculations do not ing blocks (FBB). The islands of silica tetrahedra form

1065 39#4-août-01-2256-09 1068 26/10/01, 14:57 THE CRYSTAL STRUCTURE OF FENCOOPERITE 1069

FIG. 3. The fencooperite framework in oblique projection approximately along [110], with [001] vertical, displaying the funda- 3+ mental building blocks [Si8O22] in yellow and [Fe 3O13] in red.

FIG. 4. The fencooperite structure projected along [001]. The Ba atoms are shown in or- 3+ ange, the (SiO4) tetrahedra, in yellow, and the (Fe O5) tetragonal pyramids, in red; the (CO3) polyhedra are black triangles, the Cl atoms are shown in green, and the (H2O) groups, in blue. The unit cell is outlined.

1065 39#4-août-01-2256-09 1069 26/10/01, 14:57 1070 THE CANADIAN MINERALOGIST

[Si8O22] units described as double open-branched triple rahedra cross-linked into sheets by chains of edge-shar- 3+ tetrahedra sharing vertices along [001] (Fig. 2a). The ing [(Ti,Fe )O6] octahedra (Mazzi & Rossi 1980). 2+ 3+ only FBB that vaguely resembles this is found in the Orthoericssonite and ericssonite, BaMn 2Fe Si2O7 crystal structure of synthetic NaBa3Nd3[Si2O7][Si4O13] (O,OH)2, contain composite sheets of [Si2O7] groups 3+ (Malinovskii et al. 1983). In that structure, the [Si4O13] cross-linked by (Fe O5) square pyramids interlayered group would be equivalent to one-half of the doubled by sheets of (MnO6) octahedra and Ba atoms. The dis- FBB in fencooperite i.e., a single-layer open-branched tribution of silica tetrahedra and the five-fold coordina- triple tetrahedra. The other unique FBB consists of three tion of iron in fencooperite seem to be a crystal-chemical 3+ [Fe O5] tetragonal pyramids with one of the O atoms hybrid of the structures found in these other phases. In 2+ common to all three polyhedra, [Fe3O13] (Fig. 2b). This gillespite and pellyite, individual polyhedra of Fe in pinwheel trimer cross-links the [Si4O13] islands form- four-fold coordination with oxygen cross-link sheets and ing a framework structure (Fig. 3). The framework sup- chains of silica tetrahedra, whereas in (titan)taramellite ports intersecting tunnels in all three crystallographic the chains of Fe3+ having six-fold coordination with directions, which are filled by Ba and Cl atoms and by oxygen cross-link isolated rings of silica tetrahedra. H2O and CO3 groups (Fig. 4). The H2O group (OW10) Perhaps the most closely related mineral to fencooperite has site symmetry 3m, which contradicts the point-group from the point of view of crystal chemistry would be 3+ symmetry 2mm for a H2O group, necessitating disorder orthoericssonite, with Fe in five-fold coordination and of the H atoms. This disorder is manifested in the ap- forming an integral part of the quasi-silicate sheet. It parent lack of hydrogen bonds. There are three O4 at- should be noted that the five-fold coordination noted in oms at a distance of 3.09 Å, which is too long for any fencooperite is also observed for Fe2+ in graftonite appreciable hydrogen-bonding effects. Inspection of (Kostiner & Rea 1974), but it seems that in the Trumbull Figure 4, in conjunction with Table 2, shows the close Peak locality, the paragenesis is such that this coordina- proximity of symmetry in this structure to space-group tion complex is primarily, and probably entirely, domi- symmetry P6øm2 (origin at ⅔, ⅓, 0), a supergroup of nated by Fe3+. P3m1. This similarity explains the diffraction symme- try 6/mmm noted earlier in this paper. The two CO3 ACKNOWLEDGEMENTS groups are what lowers the rotation symmetry from a 6- fold axis to a 3-fold axis, and the displacement of these The author thanks Dr. F.C. Hawthorne, University two groups to unequal distances above and below z = 0 of Manitoba, for use of the fully automated CCD- destroys the inversion point, resulting in the space group equipped four-circle diffractometer. I also thank Gail P3m1. Space group P31m requires reorientation to a Dunning for providing specimens and information non-standard, larger unit-cell. regarding the occurrence and associated species of McDonald et al. (2000) discussed the unusual oc- fencooperite. The manuscript was improved by the com- currence of five-coordinate Ti4+ with a distorted square ments and suggestions of two referees, Drs. Franco pyramidal geometry in yoshimuraite. In fencooperite, Demartin and Herta Effenberger, Associate Editor Carlo the same unusual polyhedron is observed with Fe3+. The Maria Gramaccioli and Robert F. Martin. This research electronic configuration d5 for Fe3+ has five 3d elec- was made possible by the Canadian Museum of Nature. trons. In this distorted polyhedron, the electrons will most likely be high spin, with three unpaired electrons REFERENCES (Cotton & Wilkinson 1980). There is little doubt that 3+ this electronic configuration in the [Fe O5] tetragonal BRESE, N.E. & O’KEEFFE, M. (1991): Bond-valence parameters pyramids gives rise to the striking optical properties of for solids. Acta Crystallogr. B47, 192-197. fencooperite. Fencooperite, like Fe-rich tourmaline, is strongly dichroic, with >> in plane-polarized light. DUNNING, G.E. & COOPER, J.F., JR. (1999): Barium silicate minerals from Trumbull Peak, Mariposa County, Califor- A much rarer optical property is its dichroism in unpolar- nia. Mineral. Rec. 30, 411-417. ized light, like that of Fe-bearing cordierite and zoisite. Although the fencooperite structure is unique, it is COTTON, F.A. & WILKINSON, G. (1980): Advanced Inorganic worth comparing structural features with those of some Chemistry: A Comprehensive Text (4th ed.). John Wiley & 2+ of its coexisting phases. Gillespite, BaFe [Si4O10], has Sons, New York, N.Y. corrugated sheets of (SiO4) tetrahedra that are cross- 2+ HAZEN, R.M. & FINGER, L.W. (1983): High-pressure and high- linked by (Fe O4) polyhedra with a square-planar geometry (Hazen & Finger 1983). Pellyite, Ba CaFe2+ temperature crystallographic study of the gillespite IÐII 2 2 phase transition. Am. Mineral. 68, 595-603. [Si6O17], has chains of six-membered rings of (SiO4) 2+ tetrahedra cross-linked into sheets by (Fe O4) tetrahe- NTERNATIONAL ABLES FOR RAY RYSTALLOGRAPHY VOL 3+ I T X- C , . dra (Meagher 1976). Titantaramellite, Ba4(Ti,Fe )4 C (1992): Kluwer Academic Publishers, Dordrecht, The [B2Si8O27]O2Cl, has four-membered rings of (SiO4) tet- Netherlands.

1065 39#4-août-01-2256-09 1070 26/10/01, 14:57 THE CRYSTAL STRUCTURE OF FENCOOPERITE 1071

KOSTINER, E. & REA, J.R. (1974): Crystal structure of ferrous MCDONALD, A.M., GRICE, J.D. & CHAO, G.Y. (2000): The , Fe3(PO4)2. Inorg. Chem. 13, 2876-2880. crystal structures of yoshimuraite, a layered BaÐMnÐTi silicophosphate, with comments on five-coordinated Ti4+. LE PAGE, Y. (1987): Computer derivation of the symmetry Can. Mineral. 38, 649-656. elements implied in a structure description. J. Appl. Crys- tallogr. 20, 264-269. MEAGHER, E.P. (1976): The atomic arrangement of pellyite: Ba2Ca(Fe,Mg)2Si6O17. Am. Mineral. 61, 67-73. MALINOVSKII, YU.A., BATURIN, S.V. & BONDAREVA, O.S. (1983): A new island silicate radical [Si4O13] in the struc- ROBERTS, A.C., GRICE, J.D., DUNNING, G. & VENANCE, K.E. 3+ ture of NaBa3Nd3[Si2O7][Si4O13]. Sov. Phys. Dokl. 28, 809- (2001): Fencooperite, Ba6Fe 3Si8O23(CO3)2Cl3¥H2O, a 812. new mineral species from Trumbull Peak, Mariposa County, . Can. Mineral. 39, 1059-1064. MAZZI, F. & ROSSI, G. (1980): The crystal structure of taramellite. Am. Mineral. 65, 123-128.

MATSUBARA, S. (1980): The crystal structure of orthoerics- Received February 21, 2001, revised manuscript accepted July sonite. Mineral. J. 10(3), 107-121. 5, 2001.

1065 39#4-août-01-2256-09 1071 26/10/01, 14:57